The use of compacts comprising essentially a source of carbon (typically graphite) and suitable metals and their alloys as reaction volumes for diamond synthesis under high-pressure, high-temperature (HPHT) conditions is well known in the art. The metals are typically selected from iron, nickel, chromium, manganese and cobalt, and/or alloys of these, and they are thought to have a dual solvent/catalyst function under UHPT conditions, serving to dissolve the carbon source and promote diamond growth. Typically, small diamond particles are disposed throughout such reaction volume compacts (RVC) in order to seed diamond growth. An essential element of using such RVC's is the heating of said compacts at elevated temperatures (typically >500 degrees Celcius (deg C.), and more typically > about 900 deg C. but < about 1200 deg C.) in a chosen atmosphere and/or relatively low pressure environment in order to reduce the oxygen and nitrogen content within the compact prior to the diamond synthesis process.
As taught by EP0737510 B1 (Sumitomo), there are certain advantages to manufacturing the RVC using granules comprising coated diamond seed particles, where the coating comprises at least one type of solvent metal powder, a source of carbon for the diamond growth (specifically a graphitic source), and possibly an organic binder material. Such granules may be compacted so as to yield an RVC in which the fine diamond seeds are arranged in a regular array or at least separated by a certain minimum distance from each other. Using such compacts to synthesize diamond, as is well known in the art and discussed in EP0737510 B1, has the potential to yield an increased quantity of high quality diamond than would be the case if the seed diamond were randomly distributed throughout the compact.
One of the embodiments of the invention disclosed in EP0737510 B1 is a granule form where the seed diamond is coated or clad with metal using plating or Chemical Vapour Deposition (CVD) methods and wherein the coated seed is subsequently coated with a mixture of metal, graphite and binder material in various combinations. One advantage of coating or cladding the seed diamond with metal using a plating or CVD method prior to further coating using a fluidized bed method is that the resultant seed diamond/metal composite particle can be made significantly larger and heavier than the seed diamond itself, thereby facilitating the subsequent fluidized bed coating or other coating method. This is especially true where the seed diamond needs to be as small as possible, typically less than about 0.2 mm in diameter. It is desirable to use as small seeds as possible in order to minimize the metallic inclusion uptake within the grown diamond crystals, as is the case in the commercial manufacture of high quality diamond grit less than about 2 mm in diameter.
Published patent application US20050136667 also discusses the cladding of seeds with a thick layer of solvent/catalyst metals, such as Co, Fe and Ni, in order to prevent direct contact between seeds and the surrounding carbon source. It is asserted in this reference that coatings of Ni or Ni alloys are preferred.
Patent JP59164607 (Showa Denko) claims that coating the diamond seed with a metal coating with very low concentration of oxygen and nitrogen creates the potential to make extremely pure diamond particles with reduced metallic inclusions.
An alternative method for achieving an RVC in which the seed diamond distribution is controlled, rather than random, is disclosed in the prior art. Specifically, patents EP0528195 B1 (Sumitomo) and U.S. Pat. No. 4,547,257 (Showa Denko) disclose methods pertaining to the placement of seed diamonds in two-dimensional arrays onto the surfaces of disc-like layers of various kinds, then arranging the discs on top of one another and compacting the resulting stack in order to create a compact in which the seed diamonds are arranged in a relatively ordered, non-random way relative to one another. The benefits of doing this are similar to those discussed above, with respect to the increased yield of high quality diamond arising from the synthesis process.
One significant disadvantage of coating or cladding the seed diamonds with a sufficiently thick layer of metal prior to disposal of the seeds within a synthesis reaction volume by any method such as those disclosed above is that the metal layer deleteriously perturbs the early stage growth of the diamond on the seed, as compared to the traditional approach where the seed diamond is not surrounded by predominantly metal layer but by a mixture of metal and graphite. This perturbation of the initial growth conditions is likely to alter the required synthesis process control conditions and/or the properties of the resultant grown diamond product. If the control conditions are not altered to compensate for the coating or cladding around the seeds, the resultant grown diamond is likely to be of inferior quality.
A need exists for the methods of the prior art not burdened by the perturbation of the initial growth conditions, as hereinbefore described.